History of Cells
Cells have often been called the building blocks of life, and are indeed found to the basis of everything we generally classify as life (viruses are their own special category). Cells were discovered by a British scientist named Robert Hooke in 1665. The name was originally applied to cells found in a cork! These "cells" looked like little compartments, similar to the living quarters of monks which were called cells. Much later on, two German scientists discovered the connection between life and cells. Matthias Schleiden discovered plants are made of cells, while Theodor Schwann realized animals are composed of cells. We have since learned that bacteria are composed of single cells (unicellular), while more complex forms of life are multicellular.
The ideas of Schleiden and Schwann helped shape modern biology. From them we have derived a set of rules for living things. This is known as the "cell theory."
- An organism is composed of a least one cell. All living things, from unicellular bacteria to complex humans have at least one cell.
- The cell is basic building block of the organism. It is the lowest organizational structure of all living things.
- All cells come from previous cells. Cells are made from other cells during a process known as cellular division.
All living things follow these rules (though there are controversial exceptions to these rules).
Types of Cells
There are two types of cells that we see in living things. A prokaryotic cell lacks a nucleus and is not as complicated as a eukaryotic cell. Prokaryotes, organisms consisting of prokaryotic cells, are usually unicellular. Eukaryotes, on the other hand, normally consist of many cells working together. Both structures are surrounded by a membrane, but a eukaryote has organelles. Organelles are sub-units inside the cell with their own membrane, and a specific function (such as producing food for the cell). The largest organelle in a eukaryote is the nucleus, which acts as a control center for the cell.